This application claims priority from German Patent Application DE 101 44 591.1, filed Sep. 11, 2001, which is incorporated by reference in its entirety.
The invention relates to a circuit arrangement for regulating a voltage and to generate, from a given input voltage, an output voltage that does not exceed a given maximum. The circuit arrangements of interest in the present context find application in cases where any kind of consumer devices require an interruption-free voltage supply and where the input voltage of the circuit arrangement may be higher than the maximum permissible voltage of the consumer devices. For example, circuit arrangements of this kind are used to limit the rise in voltage during the charging process in battery-powered systems or when the power is withdrawn through an inverter, or during the so-called load dump when power is supplied by a generator.
The state of the art includes the following circuit arrangements, some of which are described in xe2x80x9cHalbleiter Schaltungstechnikxe2x80x9d, Tietze, Schenk, ISBN 3-540-19475-4.
In principle, one has to distinguish between two operating states of the circuit arrangement:
1. The magnitude of the supply voltage is lower than the maximum permissible operating voltage of the consumer devices. In this case, no measures need to be taken to regulate the voltage.
2. The magnitude of the supply voltage is higher than the maximum permissible operating voltage of the consumer devices. In this case, the output voltage of the circuit arrangement has to be lower than or equal to the maximum permissible operating voltage of the consumer devices.
One possibility to protect consumer devices from a voltage that is higher than the maximum voltage permissible for the given consumer devices is to temporarily separate the consumer devices from the excessive voltage. During the temporary separation, the consumer devices are supplied with power from an energy-storage device such as an accumulator or a capacitor, to ensure that the devices continue to operate without interruption.
The foregoing solution has the disadvantage that energy-storage devices such as accumulators or capacitors have a limited capacity. Also, if capacitors are used, large charging currents will occur at the time when the circuit is turned on. Accumulators, on the other hand, require the addition of a charger device. Therefore, the concept of separating the consumer devices from the supply voltage is feasible only for consumer devices with low power consumption and/or if the periods when the supply voltage exceeds the limit are relatively short. However, even if these conditions are met, the arrangements just described still suffer from the main drawback that they involve circuits of considerable complexity and require a considerable amount of space.
Another possibility to protect consumer devices from excessive supply voltage levels is to take appropriate measures already in the design stage of the consumer devices, so that they will be able to tolerate the maximum possible excess voltage. This means that all components of the consumer device circuits have to be selected or designed to withstand the maximum anticipated voltage level, which in most cases entails a higher cost of the device as well as higher power losses, for example because semiconductor elements for higher voltages as a rule have a poorer conductance. For these reasons, the last mentioned solution is feasible only if the maximum levels of the over-voltages exceed the normal operating voltage by no more than a small amount.
A third possibility is offered by so-called clamping circuits, i.e., special components such as zener diodes, varistors, or suppressor diodes, which dissipate the energy contained in the difference between the over-voltage and the maximum operating voltage into heat. However, the fact that they are heat generators also represents the main drawback of these devices. The aforementioned components can only absorb a limited amount of dissipated energy and are therefore usable only for short-term and low-energy over-voltages.
As a forth possibility, it is possible to use so-called longitudinal regulator circuits, which are known in the form of discrete circuits as well as integrated circuits. They have the disadvantage that they work with a considerable loss of energy even in an operating state where the input voltage is below the maximum permissible operating voltage of the consumer devices. Even integrated circuits with minimized loss characteristics, so-called low-drop regulators, still have a voltage drop of about 200 mV across the component.
It is the foregoing and various other drawbacks of this prior art which the present invention seeks to overcome by providing a voltage regulator circuit that works with a minimum amount of energy loss when the input voltage is less than or equal to the maximum permissible operating voltage of the consumer devices, and which is further distinguished by low component cost and low design complexity to achieve the regulating function when the input voltage is higher than the maximum permissible operating voltage of the consumer devices.
The voltage-regulating circuit according to the present invention includes a longitudinal regulator circuit and a charge-pumping circuit. The longitudinal regulator circuit contains a transistor, a resistor, and a zener diode, while the charge-pumping circuit has a resistor, a diode, a capacitor, and a switched voltage source. The resistor, the capacitor, and the switched voltage source of the charge-pumping circuit are connected in series between the output of the transistor of the longitudinal regulator circuit and a reference potential, e.g., chassis ground. The anode of the diode of the charge-pumping circuit is connected to the mid-point between the resistor and the capacitor of the charge-pumping circuit, while the cathode is connected to the controlling signal input of the transistor of the longitudinal regulator circuit.
In an embodiment of the inventive circuit, the transistor of the longitudinal regulator circuit is a bipolar transistor, a MOS-FET (Metal Oxide Semiconductor Field Effect Transistor), or an IGBT (Insulated Gate Bipolar Transistor).
The inventive circuit may further include a switch between the controlling input of the transistor in the longitudinal regulator circuit and the reference potential, so that the circuit can be turned off.
In a further embodiment, the zener diode of the longitudinal regulator circuit can be replaced by a circuit element that is supplied and controlled by a feedback voltage that is tapped off a voltage divider from the output of the voltage-regulating circuit.
As described above in connection with the prior art, one has to distinguish again between two operating states:
1. The magnitude of the supply voltage is lower than the maximum permissible operating voltage of the consumer devices. In this case, no measures need to be taken to regulate the voltage. The transistor of the longitudinal regulator circuit is therefore not being operated as a regulating element, but as a switch that is controlled by the switched charge-pumping circuit. Thus, a power loss occurs only due to a conductance-related loss in the transistor rather than to a loss in the longitudinal regulator circuit. The added design complexity of a charge-pumping circuit is small, given that a clock signal similar to the one provided by the charge-pumping circuit is already available in many applications.
2. The magnitude of the supply voltage is higher than the maximum permissible operating voltage of the consumer devices. In this case, the circuit arrangement according to the invention works like a state-of-the-art longitudinal regulator circuit. The charge-pumping circuit still operates and generates an additional, albeit insignificant, contribution to the power loss. The benefits of the longitudinal regulator circuit are preserved, e.g., it is unnecessary to separate the input voltage, and only the components of the protector circuit have to be designed to tolerate the over-voltage.